1. Field of the Invention
The present invention relates to a centrifugal rotating disc assembly for a biological waste water treatment apparatus including one or more rotating discs arranged in tandem or in parallel and rotatively driven while their lower halves are immersed in waste water for its purification with a high efficiency.
2. Description of the Prior Art
The rotating disc-type waste water treatment apparatuses for purifying waste water have been known, whose principle is based on the fact that the bacteria, microorganisms and algae propagated on the surfaces of rotating discs whose lower halves are immersed in the waste water take up and digest the nitrogen or phosphorus in waste water as nutrient. Various waste water treatment apparatuses of the aforenoted rotating type have been proposed. It is generally required for the treatment apparatus to provide great areas for attaching the microorganisms to improve its purification without increasing outer diameters of the discs and axial lengths. It is also required for the apparatus to be light weight, compact and durable for many years and to be driven by as little driving power as possible.
In some instances of the prior art, circular discs formed with recesses and protrusions on both their surfaces have been proposed for the purpose of increasing the surface areas to improve purification. However, there has been a tendency of such rotating discs to require increased driving power. In order to make the rotating discs light, attempts have been made to produce the discs of foamed polystyrene, waterproof plywood and the like. However, the discs made of these materials may be inferior in durability.
It has been suggested to provide a centrifugal rotating disc-type biological oxidation treatment apparatus including two parallel thin discs having at its center a suction opening and impeller blades therebetween to form a rotating dual disc in the form of a centrifugal impeller. Such a plurality of the dual discs are fitted on a main shaft to form a multiple stage dual disc assembly. This rotating disc assembly is compact and light weight and has surface areas for attaching the generated microorganisms which are wide in spite of its short axial dimension. The rotating disc assembly is made in the form of centrifugal impellers which facilitate with the aid of the centrifugal force the attaching and aerating of the microorganisms and natural discharge of accumulated microorganisms from the surfaces of the rotating disc assembly. In addition, the rotating disc assembly is driven with less driving power because of its low hydraulic frictional resistance resulting from its relatively simple configuration. In order to achieve a higher purification efficiency, however, it is desired for such a rotating disc assembly to increase the surface areas of the discs.
It has been suggested in the United States to provide rotating discs of double walls having a number of concentric corrugations for the purpose of increasing the surface areas. However, these rotating discs are apt to become anaerobic because of their low permeability and have a disadvantage in that removed microorganisms tend to clog on the corrugations of the discs.
It has been suggested to drive such rotating discs by pneumatic power while utilizing it for the exposure of the waste water to air, thereby improving the purification efficiency. However, the suggested pneumatical drive devices are generally of low efficiency in driving apparatus. A pneumatically driven rotating disc-type waste water treatment apparatus has been proposed, for example, by U.S. Pat. No. 3,886,074 wherein rotating discs are formed at their outer cirumferences with a number of closed inner ended pockets opening opposite to the rotating direction and are rotatively driven by the buoyancy of bubbling air trapped in the pockets which has been jetted through a number of nozzles into the waste water and rising therethrough, while the air jetted from the nozzles is utilized for the exposure of the waste water to air. In this driving system, only buoyancy of the air in the waste water serves as a driving power for the treatment apparatus.